The preparation and purification of 4,4-bis(alkyl)-4H-cyclopenta[2,1-b:3,4-b:]dithiophenes has recently attracted much attention as such molecules are useful building blocks in e.g. organic electronics.
Brzezinski et al. (Synthesis 2002, 8, 1053-1056) have proposed a three-step approach to 4H-cyclopenta[2,1-b:3,4-b′]dithiophen-4-one.
Coppo et al. (Macromolecules, 2003, 36, 2705-2711) reduced 4H-cyclopenta[2,1-b:3,4-b′]dithiophen-4-one with hydrazine according to a Huang-Minlon modification of the Wolf-Kischner procedure, thereby obtaining 4H-cyclopentadithiophene. They then performed a dialkylation with alkyl halogenides under basic conditions resulting in 4,4-bis(alkyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophenes.
Zotti et al. (macromolecule, 2001, 34, 3889-3895) synthesized 4,4′ asymmetrically disubstituted 4H-cyclopentadithiophene by performing two separate alkylation steps on 4H-cyclopentadithiophene with two different alkyl halogenides under basic conditions.
This overall procedure toward 4,4-R1R2-4H-cyclopenta[2,1-b:3,4-b′]dithiophenes wherein R1 is different from R2 requires 6 synthesis steps and requires the difficult separation of mono and disubstituted 4H-cyclopentadithiophene. Also, the presence of mono-substituted 4H-cyclopentadithiophene as an impurity, even in small amounts, is likely detrimental to it use in optoelectronic applications because the hydrogen at the 4 position is foreseen as being easily oxidized. Furthermore this 6 steps synthesis involves the use of the highly toxic hydrazine.
There is therefore a need in art for a new synthetic route toward 4,4′ disubstituted 4H-cyclopentadithiophene and especially asymmetrically disubstituted 4H-cyclopentadithiophene.